1. Field of the Invention
This invention relates to a system for detecting the position of the side edge of a web, and more particularly for constantly detecting the position of the side edge of a travelling web and correcting the lateral deviation of the travelling web by controlling a web feeding means based on the result of the detection.
2. Description of the Prior Art
On a rotary press, it is normally required to keep the roll of printing paper running at a fixed position without lateral deviation with respect to the rotary press. This is done by holding the roll of printing paper to run at a correct position, for example, by adjusting the position of a feeding means such as guide rollers of the travelling web. One end being fixed and the other end being free to move horizintally based on the result of the detection of the position of the side edge of the roll of printing paper using the side edge itself as the detecting point.
Conventionally, such a system is already known which comprises 2 units of photoelectric devices. One unit is so installed as to emit light on a designated position referred as the first position and which is located slightly inward of the datum side edge of a travelling roll of paper. The datum side edge being defined as the position of the side edge of a web while it is run at a correct position. This unit detects whether the light passes through said first position or not. The other unit is installed so as to emit light to another designated position referred as second position which is located slightly away from the datum side edge in the direction contrary to the web side. The second unit detects whether the light passes through the second position or not. The position of the side edge of the web is detected based on the results of the detection by both photoelectric devices. The two units of photoelectric device are in many cases intalled side by side in a plane parallel to the surface of the travelling roll of paper and in a direction perpendicular to the direction of travel of the paper. However, with this arrangement, it is difficult to obtain accurate detection due to the fact that the setting of the units at the first position and second positions is dependent on the size of the photoelectric device itself.